The present invention is related to an electrochromic device and, more particularly, to such a device conceived for modulating light transmitted or reflected by equipment items or apparati such as windows, display cases, windows or rear view mirrors for automobiles, electrooptic shutters, display screens, or others.
Electrochromic devices consisting of a liquid or solid layer of an electrolytic material confined between two transparent electrodes are known which, as the result of the application of a voltage between the two electrodes, take on a coloration due to the formation of a metallic layer on one of the electrodes or to the insertion of ions into an oxide layer. Thus, by varying electrically the optical density of the device, it is possible to modulate the transparency or the reflectance of the equipment items mentioned above for adaptation to variations in the ambient lighting, for example.
In particular, from French Patent Application No. 91 00223, filed Jan. 10, 1991 by the applicants of the present application, an electrochromic electrolytic material is known consisting of a homogeneous solution of at least one organic solvent and of at least one salt of an electrodepositable metal, the solution comprising, in addition, at least one organic acid and at least one salt of a non-electrodepositable metal facilitating the dissolution of the electrodepositable metal salt. As an example, this material can then be constituted from bismuth bromide in solution in gamma-butyrolactone, and comprises, in addition, lithium bromide to facilitate the solubilization of bismuth bromide. Such a material exhibits an improved thermal stability and permits the formation of absorbing and/or reflecting metal deposits in electrochromic devices for modulating light of large size and simple structure.
Such a device generally exhibits the structure represented partially in perspective in FIG. 1 of the appended drawing. This figure illustrates that the device takes the form of cell 1 comprising a layer of electrochromic material 2 confined between two plates 3, 4 in a transparent material such as glass, which plates carry on their two opposing faces conducting layers 5, 6, respectively. A joint 7 of an adhesive material, for example, laterally confines electrochromic layer 2, assuring the assemblage of plates 3, 4 with maintenance of a predetermined space (several tens of .mu.m) between them. Conventionally also, the application of an electrical voltage to the two conducting layers 3, 4 is assured by elongated and parallel electrodes 8, 9, respectively, disposed on the conducting layers 5, 6, respectively, along two opposite edges of the cell. Electrodes 8, 9 are prepared from a material of much higher conductivity than that of conducting layers 5, 6 generally comprised of the oxide of tin and/or indium (ITO) and prepared by deposition under vacuum. Electrodes 8, 9 thus improve the uniformity of the distribution of an electrical voltage input delivered from a source (not shown) upon the entire layer of electrochromic material.
One first difficulty that one encounters in the use of electrochromic cells resides in the necessity to assure a rapid darkening of the cell, capable of following the fluctuations of the ambient light, brutal and glaring when, for example, the headlights of a vehicle strike to flash the reflecting surfaces of a rear view mirror equipped with such a cell. There currently exists a need for electrochromic devices having, from this point of view, a very short response time, less than that possible to obtain in the cell with two electrodes disposed as shown in FIG. 1.
Among the other difficulties encountered in assuring good operation of a cell of the type shown in FIG. 1, one encounters those derived from the existence of parasitic electric currents flowing transversely through the cell from one electrode to the other in parallel to the different layers constituting this cell. One ascertains experimentally the existence of these currents which are added to the current perpendicular to the conducting layers which are necessary for the generation of the electrochromic effect. The parasitic currents cause the migration of ions from one side of a cell to the other, when one uses the input configuration with two electrodes shown in FIG. 1. Although these parasitic currents have hardly any immediate visible effects, they cause over a long time, through a cumulative effect, an asymmetry of coloration of the cell with applied voltage; coloration which is then stronger in the vicinity of one of the electrodes whereas one seeks, on the contrary, a uniformity as good as possible of this coloration.
Another difficulty involves the deterioration over time of one of the conducting layers 5, 6, the one on which a metallic deposit is brought about which assures the coloration of the cell. Indeed, physiochemical effects resulting from such deposits on the conducting layer cause a diminution of its conductivity. This diminution affects only one of the two conducting layers; the distribution of current in one cell with two electrodes of the type shown in FIG. 1 is affected in an asymmetric manner, which reinforces again the harmful effects of the parasitic currents mentioned above in the matter of losses of uniformity of the coloration of the cell under voltage.
One observes again in the cells with two electrodes of the same type a much weaker coloration in the center of the cell than that which one observes in the vicinity of the electrodes; this phenomenon being much more pronounced as the distance separating electrodes 8, 9, and therefore the width of the cell, increases.
Besides the problems of non-uniformity of coloration mentioned above, the current technology of electrochromic cells makes apparent various other needs which are not yet perfectly satisfied. The operation of such a cell requires the consumption of a certain electrical power. It appears desirable to conceive cell structures permitting reduction thereof to a minimum. In another connection, many current or potential applications of electrochromic devices concern automobile vehicles. There exists a need for such devices incorporating the possibility of electrical heating permitting a time of defrosting transparent plates 3, 4 and a reheating of the electrochromic layer to an adequate operating temperature. In this regard, it is clear that the cell with two electrodes of FIG. 1 does not permit passing a current from one side of one of the conducting layers 5, 6 to the other to heat it by the Joule effect. Document EP-A-0408427 describes an electrochromic pane comprising a third electrode enabling such heating, but only one of the two conducting layers.
The present invention therefore has a purpose of preparing an electrochromic device with a short darkening time and a coloration under voltage of improved uniformity.
The present invention also has a purpose of preparing such a device with reduced power consumption to obtain the electrochromic effect.
The present invention also has a purpose of preparing such a device incorporating means of heating of improved efficacy.